1. Field of the Invention
The present invention relates to a digital radio transmission system constituting a first terminal office, a second terminal office, and at least one drop/insertion office located therebetween, and more particularly a system which includes means for effectively performing line switching from a main line to a protection line when a fault occurs in one of the main lines.
In a digital and large capacity transmission system, a drop/insertion office is required to drop and insert transmission signals with respect to a local transmission system.
Usually a plurality of drop/insertion offices are employed in the digital radio transmission system, and therefore there is a high probability that various faults will occur in the system. Accordingly, it is required to recover from each fault quickly by effectively achieving the line switching from the main line side to the protection line side.
2. Description of the related Art
As explained hereinafter in detail, a prior art digital radio transmission system raises at least the following three problems related to the line switching.
(1) In the prior art digital radio transmission system, the line switching from the main side to the protection side is achieved at respective sections separately, i.e., between the first office and a first intermediate end office and between the second office and a second intermediate end office. In this case, the intermediate end offices comprising each drop/insertion office are usually separated from each other, and therefore, if a long distance line is formed between the first and second terminal offices, it is impossible to achieve the line switching from the faulty long distance line to the protection line unless the protection line is newly introduced in the system privately only for the long distance line.
(2) It may be possible to carry out the aforesaid drop and insertion for a part of the main lines, if the terminal office is used to serve as a repeater office, at the repeater office. In this case, the overall system becomes large and high in cost, since switching equipment (SW) and carrier-frequency terminal equipment (MUX) for the relay lines in the repeater office are usually large in size and expensive.
(3) It would be possible to realize the drop/insertion without using a repeater office. However this measure is not practical, since such a system requires complicated line control, and thus the system cannot be built at low cost.